1. Field of Invention
The invention relates to generating shaped electric fields for use as electrostatic lenses and other charged particle optic devices. In particular, surface currents on resistive materials to shape electric fields are employed, by means of which a charged particle beam in the adjacent vacuum or other ethereal medium is focused, deflected, or otherwise controlled or manipulated. "Other ethereal medium" is, for most envisioned applications a high vacuum. However, for operable purposes "ethereal medium," as utilized herein, is intended to apply to mediums having substantially an infinite resistivity and through which charged particles may traverse. For the purposes of charged particle energy spectrometry, the invention is applied to improved types of energy analyzers with specific application in the field of Secondary Ion Mass Spectrometry.
2. Discussion of the Prior Art
In the prior art of electrostatic optics for manipulating the trajectories of ions (the term "ion" hereafter understood to include all charged particles such as conventional positive and negative ions, electrons, sub-atomic particles, and charged macroscopic particles such as dust grains) the required electrostatic fields are generated by surface charge distributions placed on appropriately shaped and located isolated metallic conducting surfaces. Such charges are placed on the surfaces by means of external voltage sources, which establish the electric potential of each isolated surface. In principle, once a surface has been charged to the required electric potential the external voltage source can be disconnected from the isolated metallic conducting device; however, in practice, leakage effects usually require that a connection to the voltage be maintained.
According to Maxwell's Equations for electromagnetic fields, a steady state electric field due to surface charges on metallic conductors such as used in the prior art, intersect the charged surfaces at right angles; hence in the vicinity of each surface the electric field is perpendicular to that surface. This restriction in the prior art that only electric fields perpendicular to their generating surfaces are produced, has been frequently recognized because often a desired field shape can be produced only by locating generating surfaces in regions where they interfere with the free passage of the ion beams, thus negating or limiting the value of the device. Attempts have been made to overcome this restriction by employing a multitude of metallic parts close to but insulated from each other in a precise mechanical array, each held at an externally fixed potential differing from the potential of its neighbors in an orderly progression. Apparatus of this type is usually expensive and difficult to fabricate, and may only partially satisfy the requirements. Specifically, in the prior art of 180.degree. deflection concentric hemispherical ion energy analyzers, a problem which results in undesirable fringe fields has been approached by making the gap between the hemispheres small and by employing guard elements in the entrance and aperture regions. Drawbacks of these techniques are reduced angular acceptance and increased mechanical complexity.